Validation and errors

How an API handler reports a bad request.

Validation

Handler parameters are validated by FastAPI against their annotations.

src/routes/api/search.py

from nexy.http import Header


async def GET(q: str, limit: int = 10, x_token: str = Header(...)):
    return {"q": q, "limit": limit}

Problem	Status	Body
missing or invalid query, header, cookie or body field	`422`	`{"detail": [{"loc": [...], "msg": "...", "type": "..."}]}`
invalid path segment (`/api/users/abc` for `id: int`)	`404`	the not-found page, or JSON for `Accept: application/json`

A path segment that cannot be converted is a 404 rather than a 422: a dynamic route captures any URL, and a URL that cannot be converted does not exist.

The 422 body is FastAPI's standard shape, so existing clients and generated SDKs keep working.

Raising errors

src/routes/api/items/[id].py

from nexy.http import HTTPException


async def GET(id: int):
    item = find(id)
    if item is None:
        raise HTTPException(status_code=404, detail="No such item")
    return item

Client sends	Response
`Accept: application/json`	`{"detail": "No such item"}` with status `404`
`Accept: text/html`	the error page
`Accept: /`	the error page

Set Accept: application/json in API clients. Browsers keep getting pages.

Unhandled exceptions

An exception that your handler does not catch answers 500, with the same HTML or JSON choice.

Custom responses

Return a response object when you control the body:

from nexy.http import JSONResponse


async def POST():
    return JSONResponse({"error": "conflict"}, status_code=409)

Next steps

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